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# Water Bottle Rockets Essay

Introduction

A water bottle rocket is essentially that; a bottle modified in the image of a rocket then filled with a select amount of water that is pressurised and launched into the air due to the forces pushing the rocket upwards from the launcher. When the completed water bottle rocket is sitting on the launcher, the force of the surface of the launcher pushes the rocket up whilst gravity drags it down. As the fluid inside the rocket is pressurized, the forces become unbalanced and release the clamps that are holding back the rocket. The fluid will then be expelled through the small opening at the bottom of the rocket (the bottles nozzle) at a fast rate in one direction and therefore providing a lot of thrust into the other direction, allowing the rocket to propel upward. This force will continue to thrust upwards until the last of the fluid is expelled from the rocket (Moore, 2014).

To increase stability in the rocket, the centre of pressure and centre of mass should be in specific positions on the rocket. The centre of mass is to put simply, a balance point in an object. It is a uniform gravity field that averages the external forces surrounding the object to equalise the forces acting upon it, such as a balance point on a see-saw (HyperPhysics, 2000). The centre of pressure on a rocket is the average location of the pressure, which varies around the surface of an object (NASA, 2010). The fins and aspects of the rocket all contribute to the end position of the centre of mass and pressure and so the centre of mass should be as close to the middle as possible, with also the centre of pressure towards the back of the rocket, which is achieved by the use of big fins or fins that add weight to the rear of the rocket. When constructing the rockets, there are things to consider: Newtons First Law: – Objects at rest will stay at rest, or objects in motion will stay in motion unless acted upon by an unbalanced force. When the rocket is sitting in the launcher, the forces are balanced because the surface of the launched pushes the rocket up while the force of gravity forces it down.

When the water rocket is pressurised, the forces become unbalanced and thrust will provide upward direction for the rocket to follow. Newtons Second Law: – The acceleration of an object is directly related to the force exerted on the object and oppositely related to the mass of that object. The acceleration of the rocket will depend on how much thrust and force is put behind the upward strength to aid the rocket. Newtons Third Law: – For every action, there is always an opposite and equal reaction. When the rocket launched, there will be air drag and gravity pulling against the rocket, along with the upward thrust provided. Considering the design of the bottle rocket, fins play a major role in steadying and effectively pushing the rocket to increase the rockets aerodynamics.

If the fins are too far forward on the rocket it could put off the centre of mass and therefor cause the rocket to become heavily unstable. Some general tips for the fin design are that they should be thin or tapered, angled backwards and with rounded corners rather than sharp corners (Williams, 2014). This experiment is being conducted to explore the diverse variables involved with the rockets and how they function, and to analyse and explore further investigations included in the subject of the physics field that contributed to the understanding of the rockets.

Aim

The aim of this task is to research the key principles of rocket design and stability needed to develop the rockets. It is to observe the effects a range of variables has on the water rocket and how those variables affect the results gathered.

Hypothesis

I hypothesise that our rocket will reach heights to about 20m, at an acceleration rate from 8m/s to 15m/s.

Variables

The variables involved in this investigation are categorised into three sets; independent, dependent and controlled. The independent variable involved in this experiment is time. The dependent variables involved in this experiment are height, the angle of the rocket, the water level inside the rocket and the overall acceleration of the rocket. The controlled variable involved in this experiment is time again.

Apparatus

The apparatus used throughout this investigation included:
– x4 1.25L plastic Pepsi™ bottles
– x2 black garbage bags
– x1 hot glue gun
– x1 Stanley knife
– x4 manila folders
– approximately 5m of string
– 2 full rolls of clear tape
– scissors
– 500g of plasticine

Method

Rocket 1:
Step 1: First, brainstorm ideas for design of water bottle rocket (e.g. fin design, how many fins will be needed, nose cone design, parachute design, bottle type)  Step 2: Gather all materials needed to construct rocket Step 3: Begin with constructing the fins, trace design and cut them out of chosen material for the fins (for this experiment, thin cardboard manila folders were used). Step 4: After finalising fin design and construction, mark where the fins will be placed on the rocket, then attach to said rocket (for this experiment, a hot glue gun was used to attach the fins) Step 5: After Step 4: chose and finalise design for parachute (in this experiment, a circular parachute was chosen to be constructed, cute out of a black garbage bag, with a diameter of 65cm). Step 6: After finalisation of parachute, attach 8 strings of a length of 60cm at equal intervals around the circumference of the parachute, securing them with a knot so that they don’t become unattached.

Also cute a small hold in the centre of the parachute about 5cm in diameter. Step 7: Design nose cone (for this experiment, the design was a sharp cone shape created by a cut out from a manila folder, blunted with the plasticine wrapped around it (the bottom of another rocket was used to form the nose cone). Step 8: Tie the 8 strings used for the parachute in a knot at the bottom of the lengths of the string then glue them to the bottom of the rocket in the centre (the stand of the rocket). Step 9: Cut small grooves in the nose cone excess in order for a more secure fit over the parachute and rocket. Step 10: Wrap a small amount of plasticine around a golf ball to enhance additional weight to the rocket.

Rocket 1 modification:
1: Nose cone had to be altered due to impact of launches; tape was used to cover the nose cone to prevent further damage. 2: Fins were covered in tape to make them smoother and to decrease wind friction.

Rocket 2:

As for Rocket 1. Both rockets constructed were in direct image of each other, the second one was a back-up rocket if you will, in case severe damage came to the first rocket.

Rocket 2 modifications:

1: The string was changed on this rocket, as a practice launch caused the parachute to rip. The string was changed to some sort of twine; the back-up rocket was not used in the final testing.

Calculations

s= displacement (m)
v= final velocity (m/s)
u= initial velocity (m/s)
a= acceleration (m/s2)
t= time (seconds)

1st launch:
1st image:

2nd image:

3rd image:

Overall acceleration for Launch 1:

Height for Launch 1:

Height for Launch 2:

Height for Launch 3:

Continuations of Calculations are in Appendices.

Results
Table 1: 300ml
Launch 1
Angle (degrees)
64
Time (s)
2.2

66

1.44

32

2.25
Average
54.00
Average
1.96

Table 2: 400ml
Launch 2
Angle (degrees)
35
Time (s)
2.22

21

2.06

20

2.09
Average
25.33
Average
2.12

Table 3: 500ml
Launch 3
Angle (degrees)
50
Time (s)
2.72

72

2.56

50

3.25
Average
57.33
Average
2.84

Analysis of Results

Observations from launch 1 were that the height dramatically declined when the water rocket was filled with 400ml of water. The rocket went higher when more water was added to it.

Discussion

The aim of this task was to research the key principles of rocket design and stability needed to develop the rockets. It is to observe the effects a range of variables has on the water rocket and how those variables affect the results gathered. This aim was directly achieved through making aspects of the rocket a variation, for example the water level. The rocket constructed had three different water levels, which was the chosen variable to fluctuate from 300ml, 400ml and 500ml. As stated in the hypothesis, when the rocket was filled with 500ml of water it travelled to a higher level than when the rocket was filled with 300ml and 400ml. Therefore, the results are in corroboration with the hypothesis Unfortunately, when the results are viewed it can be seen that there is a huge drop in height for the 400ml launch. This was due to human error, as the rocket had a malfunction right before take-off (the nose cone fell off).

Human error can be an enormous defect in the results received as one minor slip-up from the person could result in a complete change in every result already established. Weaknesses in the experiment were organisation, how the rocket construction took up most of the assessment period, and when final testing came around, a select team member did not fully contribute to the recording of the data and so defected the entire group with their own actions. Performance data of the rocket constructed demonstrated that improvement was required in the needed added protecting of the nose cone and positioning of the golf ball to ensure the parachute was deployed successfully after launch.

Modifications made resulted in smoother take-off and landing, plus less collateral damage to the rocket. If the experiment was re-done, a new group would be requested as the group that was worked with for this investigation was horribly con-operative, with only three members essentially doing all the construction while the remaining member did not make any effort at all to contribute to the construction of the rocket, and instead chose to ignore the group to go off and do their own thing. Further investigations that could be conducted with the newfound knowledge of this exploration could include ones on how does weather affect the rockets? How can the most “perfect” rocket be achieved? And what would be the most effective materials to use when constructing the rocket?

Conclusion

In conclusion, the aim of this investigation was to explore water rocket design and other key principles of the rockets, then to observe the effects the field of variables had on said water rocket. Throughout this investigation the aim has been achieved through variation of design options for aspects of the rockets e.g. fin shape/size, nose cone shape and parachute design. Another variation observed on the rocket was water level held in the rocket to provide thrust for the rocket to travel upwards from the launcher. All in all, the investigation was successful in exploring the effects that variables had on the rockets height and weight. It was an enjoyable experiment to conduct and aside from minor human error, the exploration went without a hitch.

Bibliography
HyperPhysics, 2000. Center of Mass. [Online]
Available at: http://hyperphysics.phy-astr.gsu.edu/hbase/cm.html [Accessed 3 April 2014].
Moore, S., 2014. Design Consideration for Water Bottle Rockets. In: S. Moore,
ed. Water Bottle Rockets. Gold Coast: Helensvale, p. 1. NASA, 2010. Center of Pressure. [Online]
Available at: https://www.grc.nasa.gov/www/k-12/airplane/cp.html [Accessed 3 April 2014].
Williams, A., 2014. Bottle Rocket Design. In: A. Williams, ed. Class Notes. Gold Coast: Helensvale, p. 2.

Appendices

Continuation of Calculations:
Launch 2:
Image 2:

Image 3:

Image 4:

Image 5:

Overall acceleration for Launch 2:

Journal

Week 1

This week was used to begin brainstorming ideas for the rocket model. Variations in fin size, shape and position were discussed, along with nose cone and overall construction ideas for the rocket. It was decided at the end of the week that the fin would be a rhombus shape, 15cm long, and 12cm wide, with another, smaller fin inside of it to add weight. The nose cone would also have a rounded point.

Week 2

This week, construction of the fins and bottle began, as a team member was away for the entire week, not as much assembly of the rocket occurred, but by the end of the week, the fins had been constructed and parachute ideas had also been brainstormed. The end of the week it was decided that the parachute would be circular. A nose cone also was constructed, it being a rounded cone shape, with a blunt tip. Putting plasticine on the nose cone would make the nose cone more aerodynamic and would add more height to the initial take-off.

Week 3

By this week, construction of the rocket was well underway, with the fins and parachute finished and a backup rocket beginning to take place. All that was needed was to stabilise the rocket and have a finished product. It was also decided what variable would be tested during the testing, which would be the water level at 300, 400 and 500ml.

Week 4

This week, construction was finished, with difficulty. There were complications with the gluing of the extension that would hold the nose cone on the rocket, as it was positioned wrong on the rocket and had to be redone. A team member said that the rocket would need more weight to stabilise it and so a golf ball was covered in plasticine. Ideas were brainstormed on how the parachute would be attached to the rocket and it was decided that the knot at the end of the string would simply be glued to the back of the rocket, and this was achieved successfully.

Week 5

This week, testing began. Not many rockets, including our group, worked that well. The majority of nose cones in groups had to be remade due to them not coming off during take-off and landing, and fins had to be re-glued onto rockets as they were ripped off due to the force of thrust from take-off.

Week 6

Testing was well underway this week, but modifications still had to occur on the bulk of the rockets, even though they were becoming minor problems. This week, ¾ of the group were away at EXCITE camp and unbeknown to the rest of the group, the final member made a huge defective change to the rocket (the parachute was taken off it and remade with different string). Fortunately a backup rocket had been pre-made with a more practical parachute and that rocket would be officially used for the final testing, which would occur next week.

Week 7

This week, the final testing sessions 1 and 2 ensued. It was wonderful to see all the other groups progression from Week 2 to now, how their rockets have changed and been modified for the better. The first final test, which occurred on Wednesday 12th March for our groups’ rocket had the water level at 300ml, which produced a fairly good launch, but the angles were off balanced, as the rocket was launched on an angle, which affected the entire flight path. The second test, which happened on Friday 14th March, produced slightly better results, as we had two take-offs during the session, the first one being absolutely perfect but not being captured by the lady holding the high speed camera, and had to be redone. This would result in a fluctuation of results as when the second test occurred, right before the rocket set off from the podium, the nose cone tilted at an alarming rate, which threw off the aerodynamics of the entire rocket and negatively affected the results, contrary to the perfect initial take-off.

Week 8

This week, testing was finished and calculations became underway, finding out the acceleration and velocity took up most of the week. The assignment became imminent, work on that was essential. Not much practical things got done this week; practically all lessons were done figuring out how to calculate the velocity, acceleration etcetera.

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